- Triiodide
In chemistry, triiodide (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic
anion composed of threeiodine atoms. For some chemical compounds, triiodide indicates a salt of the namedcation with the triiodide anion. Examples include sodium triiodide,thallium triiodide , andammonium triiodide , [NH4] [I3] . Each of these compounds has a (mono)iodide counterpart. For other chemical compounds, triiodide indicates that each molecule contains three iodine atoms that are not bonded to each other, not forming the triiodide ion. Examples includenitrogen triiodide (NI3),phosphorus triiodide ,antimony triiodide , and gallium triiodide (Ga2I6). Some anions have the theoretical possibility to form either kind of triiodide.Thallium triiodide is described as thallium(I) triiodide; thallium(III) iodide is unknown.Triiodide ion
The triiodide ion is the simplest
polyiodide ; several higher polyiodides exist. In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions react with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolarsolvent s.Lugol's iodine contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution.Tincture of iodine contains significant amounts of triiodide.Formation and structure
The following endergonic equilibrium gives rise to the triiodide
ion ::I2 + I− unicode|⇌ I3−In this reaction, iodide is viewed as aLewis base , and theiodine is aLewis acid . The process is analogous to the reaction of S8 withsodium sulfide , except that the higher polyiodides have branched structures. [Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.]The ion is linear, as predicted by
VSEPR theory . A common explanation for thehypervalent bonding on the central atom involves athree-center four-electron bond . The bond lengths and angles of triiodide vary, depending on the compound. The dimensions of the tri-iodide Ia− Ib− Ic bonds in a few sample compounds are shown below:References
External links
* [http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3521/kinetic/kinetic.html Kinetic Study of the iodine-persulfate reaction]
* [http://www.solvaychemicals.us/pdf/Hydrogen_Peroxide/XX-122.pdf Determination of Hydrogen Peroxide Concentration]
Wikimedia Foundation. 2010.